Thesis: Ph. D., Massachusetts Institute of Technology, Department of Physics, 2015. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 115-124). / Our work was inspired by the need to improve the efficiency of new types of solar cells. We mainly focus on metal-semiconductor interfaces. In the CdSe study, we find that not all surface states serve to pin the Fermi energy. In our organic-metal work, we explore the complexity and challenges of modeling these systems. For example, we confirm that aromatic compounds indeed have stronger interactions with metal surfaces, but this may lead to the geometry changing as a result of the interaction. We also find that molecules that are not rigid are strongly affected by their neighboring molecules. Surface roughness will have an effect on molecules that more strongly bind to metal surfaces. This study of interfaces relates to one part of the picture of efficiency, but we also look at trying to go beyond the Shockley-Quiesser limit. We explore the idea of combining a direct and indirect bandgap in a single material but find that, in quasi-equilibrium, this does no better than just the direct gap material. This thesis hopes to extend our understanding of metal-semiconductor interface behavior and lead to improvements in photovoltaic efficiency in the future. / by Michelle Ruth Tomasik. / Ph. D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, February 2007. / Includes bibliographical references (p. 141-146). / Analysis of HETE-II data is discussed with the aim of understanding the intrinsic properties of gamma-ray bursts (GRBs). A technique is developed that allows the simultaneous estimation of source and background counts during a burst with coded aperture instruments such as the Wide-field X-ray Monitor (WXM) on HETE-II. A closely related photon-by-photon statistical bootstrap analysis is then described that can be used to compute the non-Gaussian error distribution of GRB temporal statistics. This is applied to the T90 and To.45 duration measures. The distribution of T90 has been extensively studied since the availability of the BATSE gamma-ray burst catalogs and is widely believed to be log-normally distributed. It is shown that intrinsically, GRBs may in fact have a much narrower distribution, and the wide log-normal may primarily be due to measurement artifacts. Computation of the TO.45 parameter enables the inference of redshifts through the recently discovered Liso-Epk-TO.45 relation. This in turn allows the compilation of a flux-limited sample of bursts with redshifts that is free of the observational selection effects inherent in spectroscopic catalogs. This analysis is performed for 31 WXM bursts and redshift-corrected distributions of T90 and T0.45 are computed. / (cont.) It is shown for the first time that the distribution of T0.45 can be modeled by an exponential distribution. The redshifts calculated through the Liso-Epk-TO.45 relation are also used to calculate the implied isotropic luminosities. The normalized luminosity function and redshift distribution of gamma-ray bursts are derived using the non-parametric methods of Lynden-Bell and Efron & Petrosian. The results imply strong evidence for luminosity evolution with redshift and are consistent with prior studies based on BATSE bursts. Concordance cosmology (QA = 0.7, QM = 0.3, Ho=70 km s-1 Mpc-1) is assumed throughout the analysis. Effects of log-normal errors in the redshifts are estimated using Monte-Carlo methods. Results indicate that a fraction close to 10% of GRBs are to be expected at high redshifts (> 5) in consonance with theoretical predictions of high-redshift Swift detections. / by Peter Csatorday. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, February 2006. / Includes bibliographical references (p. 145-152). / Interactions between genes in living organisms are intrinsically stochastic. This not only gives rise to phenotypic variation in clonal populations of cells, but also fundamentally limits signaling fidelity and cellular memory. Accurately predicting noise propagation in gene networks is thus crucial for reverse engineering natural networks and designing reliable synthetic genetic circuits. To determine how noise propagates through gene networks we measure, in single bacterial cells, expression variability and correlations between genes in a cascade and correlations with a constitutive gene. We find that noise in a gene is determined by its intrinsic fluctuations, transmitted noise from upstream genes and global noise affecting all genes. Our results imply that the dominant noise sources can be external to any given gene and that even for networks in which no component has significant intrinsic noise, total noise can be significant due to amplification of global fluctuations. We develop a Langevin type model that explains the complex behaviour exhibited by the noises and correlations, and reveals the dominant noise sources from the biochemical characteristics of the individual genes. The model successfully predicts the noises and correlations as the network is systematically perturbed. It also indicates that the additional information from the protein expression distributions can be used to better determine the system parameters and provides the basis for a Monte Carlo simulation method, which allows for fast, approximate simulations of the distributions. / As an extension and proof of applicability of the approach, we choose a natural network, the E. Coli lactose uptake network, to predict the dynamic behaviour of the distributions. We measure population distributions of protein numbers as a function of time, and show that prediction of dynamic distributions requires only a few noise parameters from the steady state noise measurements in addition to the rates that characterize a deterministic model. Our results show that even though noise sources are ubiquitous and network dependent, the protein distributions and even their dynamic behavior can be predicted from basic parameters, and the simplicity of the formulae brings the promise of decoding and designing the genetic networks that control the function of all living cells. / by Juan Manuel Pedraza. / Ph.D.

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Physics, 2009. / Includes bibliographical references (p. 99-102). / This thesis studies intersecting brane models, which are a class of quasi-realistic compactifications of string theory. Techniques are developed for exploring the complete space of intersecting brane models on an orientifold. The classification of all solutions for the widely-studied T6/Z2 x Z2 orientifold is made possible by computing all combinations of branes with negative tadpole contributions. This provides the necessary information to systematically and efficiently identify all models in this class with specific characteristics. In particular, all ways in which a desired group G can be realized by a system of intersecting branes (either as a subgroup or as the full gauge group) can be enumerated in polynomial time. For this orientifold we identify all distinct brane realizations of the gauge groups SU(3) x SU(2) and SU(3) x SU(2) x U(1) which can be embedded in any model which is compatible with the tadpole and SUSY constraints. We compute the distribution of the number of generations of "quarks" and find that 3 is neither suppressed nor particularly enhanced. The overall distribution of models is found to have a long tail. This tail in the distribution contains much of the diversity of low-energy physics structure. The tools developed in this thesis can be used to systematically explore the properties of a large class of string vacua, looking for patterns and correlations which may help in relating string theory to observed particle physics. / by Vladimir Rosenhaus. / S.B.

Thesis: S.B., Massachusetts Institute of Technology, Department of Physics, 2015. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 69-70). / In this work, I analyze the high-resolution spectra of 20 stars, chosen for their low metallicity [Fe/H] </~ -2.5 and proximity to the sun. Using these spectra I model the atmospheres of these stars by determing stellar parameters {Teff, log(g), [mu], [Fe/H]} and obtain also their chemical abundances for 17 elements including Fe, C, Sr, and Ba. Three of these stars are found to possess an overabundance of Carbon relative to Iron. Combining these chemical abundances with those from previously analyzed spectra from the same bright metal-poor star sample, I perform orbit determination and integration on a total of 59 metal-poor stars and extract their kinematic parameters. I also explore how these results depend on the assumed mass of the Milky Way. These chemical and kinematic results are then combined and compared with comparatively metal-rich (-2.5 </~ [Fe/H] </~ 0) samples; a conal distribution of velocity components with respect to metallicity is observed, as well as two distinct populations in eccentricity. The 59 bright metal-poor stars were identified as residing in the inner halo of the Milky Way. / by Weishuang Linda Xu. / S.B.

Thesis (B.S.) Massachusetts Institute of Technology. Dept. of Physics, 1957. / MIT copy bound with: A problem in programming the I.B.M. 704 digital computer / Vincent Louis Donlan. 1957. / Bibliography: leaves 30-31. / by Frederick Solomon Epstein. / B.S.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2007. / Includes bibliographical references (p. 131-138). / Two sets of studies are described in this thesis. The first describes studies conducted with sodium Bose-Einstein condensates (BEC) while the second focuses on the pairing of fermionic lithium-6 pairs in an optical lattice within the strongly interacting BEC-BCS regime. Common to both sets of studies is the use of a magnetically tunable Feshbach resonance to manipulate interactions between the atoms. In the first experiment, we destabilize a sodium BEC by switching its interactions from repulsive to attractive and studied the resulting dynamics. A local amplification of low momentum energetic instabilities was observed and the measured rate of amplification agreed well with theoretical predictions. For large condensates, this process depleted the condensate faster than the global inward collapse. Subsequently, I describe the major construction effort that was undertaken to convert our BEC machine to a two-species machine capable of cooling fermionic lithium-6. Upon its completion, we obtained a resonance superfluid of loosely bound 6Li pairs in the BECBCS crossover. When placed in a shallow optical lattice, long range phase coherence of this resonance superfluid was inferred from the presence of sharp interference peaks after ballistic expansion. With this observation we have obtained the first evidence of superfluidity of fermions in an optical lattice. A loss in phase coherence occurred when the lattice depth was increased past a critical value, possibly signaling a transition to an insulating state. Further preliminary explorations of this novel system is described followed by an outline of its potential for studying condensed matter phenomena like high temperature superconductivity. / by Jit Kee Chin. / Ph.D.

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Physics, 2007. / Includes bibliographical references (p. 43). / The thermal filamentation instability has been invoked to explain the formation of parallel plate waveguides in mid-latitude ionospheric plasmas during Arecibo, Puerto Rico heating experiments in 1997. The geometry of the kilometer-scale parallel plates predicted by thermal filamentation depends on the mode of the transmitted heater wave, as does the threshold to excite this instability. While plasma heating can excite small-scale irregularities via parametric instabilities, thermal filamentation is thought to produce large-scale irregularities. In Arecibo, the threshold for X-mode to induce irregularities was found to be greater than 1 V/m, while for O-mode it was on the order of mV/m. In recent plasma experiments in high-latitude ionospheric plasmas, carried out at the HAARP facility in Gakona, Alaska in summer 2005, spring 2006, and summer 2006, a weakening in ionogram traces was observed during O-mode and X-mode heating, leading to a scenario detailing the effects of thermal filamentation and short-scale irregularities caused by heating. The Gakona experiments using a high power HF heating facility and multiple diagnostic instruments shed light on the important role of the thermal filamentation instability in generating electromagnetic wave-induced plasma turbulence with a broad spectrum of wavelengths, ranging from meter to kilometer scales. / by Joel Cohen. / S.B.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2004. / Includes bibliographical references (p. 125-144). / I present high-resolution X-ray grating spectroscopy of neutron stars in low-mass X-ray binaries (LMXBs) using instruments onboard the Chandra X-ray Observatory and the X-ray Multi-Mirror Mission (XMM-Newton). The first part of this thesis concentrates on results from the subset of LMXBs with orbital periods less than an hour, known as ultracompact binaries. Previous low-resolution X-ray spectra of four systems (4U 0614+091, 2S 0918-549, 4U 1543-624, and 4U 1850-087) all contain a broad residual near 0.7 keV which had been attributed to unresolved line emission. I show that this residual is due to an incorrect model of the intervening photoelectric absorption and can be accounted for by allowing a non-standard Ne/O abundance ratio in the intervening material. I propose that there is neon-rich material local to each binary and that the mass donor is a low-mass, neon-rich degenerate dwarf in an ultracompact binary. Follow-up spectroscopy of 2S 0918-549 and 4U 1543-624 with the High Energy Transmission Grating Spectrometer (HETGS) onboard Chandra and the Reflection Grating Spectrometer onboard XMM confirms the excess neutral neon absorption. Interestingly, the Ne/O ratio of 4U 1543-624 varies by a factor of three between the Chandra and XMMobservations, supporting the suggestion that some of the absorption originates local to the binaries. I also present X-ray spectroscopy of another ultracompact binary, the accretion-powered millisecond pulsar XTE J0929-314. No emission or absorption features are found in the high-resolution spectrum of this source, and the neutral absorption edge depths are consistent with the estimated interstellar absorption. The second part of this thesis uses LMXBs as probes of the interstellar medium (ISM). / (cont.) High-resolution X-ray studies of ISM absorption features can provide measurements of the relative abundances and ionization fractions of all the elements from carbon through iron. X- ray studies also probe the ISM on larger scales than is possible in the optical and ultraviolet wavebands. I present high-resolution spectroscopy of the oxygen K-shell ISM absorption edge in seven X-ray binaries using Chandra. The best-fit model consists of two absorption edges and five Gaussian absorption lines and can be explained by the recent theoretical calculations of K-shell absorption by neutral and ionized atomic oxygen. Significant oxygen features from dust or molecular components, suggested in previous studies, are not required by the Chandra spectra. These measurements also probe large-scale properties of the ISM, placing a limit on the velocity dispersion of the neutral lines of less than 200 km s-1 and constraining the interstellar ratio of O II/O I to approximately 0.1 and the ratio of O III/O I to less than 0.1. / by Adrienne Marie Juett. / Ph.D.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Physics, 2005. / Includes bibliographical references (p. 57-58). / In this thesis, I study the process of evolution of the gene regulatory network in Escherichia coli. First, I characterize the portion of the network that has been documented, and then I simulate growth of the network. In this study, I assume that the network evolves by gene duplication and divergence. Initially, the duplicated gene will retain its old interactions. As the gene accumulates mutations, it gains new interactions and may or may not lose the old interactions. I investigate evidence for the duplication-divergence model by looking at the homology and regulatory networks in E. coli and propose a simple duplication-divergence model for growth. The results show that this simple model cannot fully account for the complexity in the real network fragment as measured by conventional metrics. / by Juhi Kiran Chandalia / S.M.